Microwave system



United States Patent MICROWAVE SYSTEM Kenneth N. Bergan, Franklin Park, Ill., assignor to Motorola, Inc., Chicago, Ill., a corporation of Illinois Application September 28, 1949, Serial No. 118,205

3 Claims. (Cl. 343-175) This invention relates generally to radio communication systems and more particularly to such systems which operate at microwave frequencies.

Much interest has been indicated in the use of microwave frequencies for point-to-point communication. The words microwave frequencies are used hereinafter to designate frequencies of 1,000 megacycles and higher, which in general fall within the bands designated as the ultrahigh frequency and superhigh frequency bands. Such high frequencies are advantageous in that large band widths are available so that a plurality of separate communications can be provided over a single channel. Microwave operation is also advantageous in that the radiated Waves at these frequencies are highly directional and can be accurately directed to a receiving point thereby providing highly efficient transmission and at the same time avoiding interference between various communication systems operating in the same general area.

Although a great amount of work has been done in developing microwave communication systems, the equipment required is relatively expensive and critical in adjustment. Oscillators for generating waves at this frequency utilize tubes such as klystrons and magnetrons which are expensive and relatively delicate so that extreme care must be taken in the use thereof.

Therefore, it is an object of the present invention to provide a simplified system for providing simultaneous transmission and reception at microwave frequencies.

A further object of this invetnion is to provide a combined transmitting and receiving system for microwaves which utilizes a single oscillator.

Another object of this invention is to provide a simplified microwave relay system in which a single oscillator produces oscillations which may be used for providing a plurality of functions at the same time.

A feature of this invention is the provision of a radio communication system providing simultaneous two-way operation in which a single oscillator operating at a microwave frequency functions as the oscillator of the transmitter and also as the local oscillator of the receiver.

A further feature of this invention is the provis1o n of a system for simultaneously transmitting and receiv ng microwaves with the transmitted and received carrier waves being at different frequencies and being modulated by subcarriers of different frequencies, so that the oscillator which is modulated to provide the transmitted wave may be used for demodulating the received wave and the received subcarriers may then be derived.

Another feature of this invention is the provision of a microwave relay system including an oscillator which can be directly modulated for providing a modulated carrier wave to be transmitted and in which the modulated wave is applied to means for demodulating a received wave.

Further objects and features will be apparent from a consideration of the following description taken in connection with the accompanying drawings in wh ch:

Fig. 1 illustrates a communication system In accordance with the invention;

Fig. 2 illustrates a modified system in accordance with the invention in which a single antenna at each terminal provides both reception and transmission;

Fig. 3 is a microwave relay system in accordance with the invention; and I Figs. 4 and 5 illustrate modified microwave relay systems.

In practicing the invention there is provided a continuous two way (duplex) microwave radio communication system. Transmission and reception are carried on simultaneously at different frequencies, with the transmitted and received waves being modulated by subcarrier waves which are at different frequencies. The transmitting equipment includes an oscillator which can be directly modulated such as klystron and magnetron oscillators which are available and are suitable for this purpose. The oscillator is modulated by one or more subcarrier waves which are individually modulated by the signals to be transmitted. The oscillator is connected to a suitable antenna which radiates the modulated carrier wave produced thereby. A receiving system is provided which may be connected to the same antenna as the oscillator, or to a separate antenna. The receiving system includes a demodulator for deriving the modulation from the received carrier wave. The carrier wave to be received is modulated by one or more subcarrier waves, the frequency of which is different from the frequency of the subcarrier waves being transmitted. The oscillator is connected through suitable attenuating means to the demodulator for providing local oscillations required thereby for demodulating the received wave. When a single antenna is used for transmitting and receiving, a filter is provided between the antenna and the receiver for eliminating the transmitted waves. The system described above can be used for relay operation with the subcarriers received from one link of the relay system being applied to the oscillator for the next link, and so on.

Referring now to the drawings, in Fig. 1 there is illustrated a radio communication system providing simultaneous two way operation in which the same oscillator provides transmission at one frequency and acts as a local oscillator for reception at a different frequency. A modulating signal derived from any source 10 is applied to subcarrier generator 11 and is used to modulate the subcarrier wave at frequency he produced thereby. The output of the subcarrier generator 11 is applied to the oscillator 12 so that the oscillator produces a Wave of microwave frequency F1 which is modulated by the subcarrier wave fro. Frequency modulation of the subcarrier waves by the modulating signal and frequency modulation of the microwaves by the combined subcarrier waves may be used. A klystron or a magnetron oscillator is suitable for providing the modulated microwave. The system disclosed, however, is not limited to use with frequency modulation. The modulated carrier wave from the oscillator 12 is applied to antenna 13 and is radiated thereby.

For receiving signals at the terminal station described, a second antenna 14 is provided. The received signals will be a microwave frequency carrier wave F2 modulated by a subcarrier wave fii. The signals from the antenna 14 are applied to the demodulator 15, and the oscillator 12 is connected through attenuator 16 to the demodulator 15 to provide local oscillations therefor. The demodulator 15 is of the superheterodyne type, and it is obvious that the carrier waves F1 and F2 must be separated by the intermediate frequency of the demodulator 15. During simultaneous transmission and reception, the oscillator 12 will be modulated by the wave fie from the subcarrier generator 11 as previously described. The output of the oscillator therefore will include this modulation and when used to demodulate the received signal, the subcarrier wave he produced by subcarrier generator 11 will be present. That is, the intermediate frequency wave produced by the converter in the demodulator will include both the received subcarrier wave fii and the transmitted subcarrier Wave in. The detector of the demodulator will also provide both the subcarrier waves fii and fio. The output of the demodulator 15 is applied to the subcarrier receiver 17 which is highly selective so that only the received subcarrier wave in will be received thereby and the transmitted subcarrier is thereby eliminated. The subcarrier receiver recovers the original modulating signal from the received subcarrier wave.

The terminal station just described may communicate with another station which is identical therewith, as illustrated in Fig. 1. The second station includes a modulating signal source 20, subcarrier generator 21 operating at frequency frr, oscillator 22 operating at frequency F2, transmitting antenna 23, receiving antenna 24, demodulator 25, attenuator 26 and subcarrier receiver 27. The demodulator is arranged to receive the frequency Fr transmitted by the antenna 13, while the oscillator 22 will produce a carrier wave of the frequency Fz being received by the demodulator 15. The oscillator 22 provides oscillations to the demodulator 25 in the manner fully described above.

In Fig. 2 there is disclosed a microwave radio communication system which is generally similar to that of Fig. 1. In this system a plurality of modulating sources 30 are provided which individually modulate the subcarrier generators 31. The oscillator 32 is modulated by the waves fro and in from the subcarrier generators and applies the modulated carrier wave F1 to the antenna 33. A single antenna 33 is used for transmission and reception and is connected to the demodulator 34 of the receiver through filter 35 which passes the received frequency F2. The filter may be of the cavity type and prevents the relatively strong transmitted signal F1 from being applied to the receiving system. Oscillations are applied from the oscillator 32 to the demodulator 34 through the attenuator 36. Sufficient signals may be transmitted through the filter 35 so that the path through the attenuator may be eliminated. Connected to the output of the demodulator are a plurality of subcarrier receivers which may be selective to derive the subcarrier frequencies frz and fix being received. The subcarrier receivers 37 provide the separate modulating signals from the subcarrier waves.

The second terminal of the system in Fig. 2 may be identical in all respects to the terminal just described with the demodulator 40 responding to the frequency F1 of the oscillator 32 and the subcarrier receivers 41 being individually responsive to the frequencies fro and frr of the subcarrier generators 31. Similarly the oscillator 42 will produce the frequency F2 to which the demodulator 34 is responsive and the subcarrier generators 43 will individually provide the subcarrier frequencies frz and f1; to which the subcarrier receivers 37 are responsive.

In Fig. 3, a relay system is illustrated in which the terminal equipment is generally similar to that illustrated in Fig. 2. To simplify reference thereto, the terminal stations are indicated A and B and the relay station is indicated as C. The terminal stations A and B may be identical to the terminal stations illustrated in Fig. 2 and therefore will not be described in detail. The components illustrated are designated by the same reference numbers used in Fig. 2. At the terminal station A, the oscillator 32 provides a carrier wave F1 modulated by subcarrier fro which is transmitted by the antenna 33. The wave F4 received by the antenna 33 is applied through filter 35 to the demodulator 34 to which local oscillations are applied from oscillator 32 through attenuator 36. Similarly the terminal station B includes oscillator 42 for applying carrier wave F2 modulated by subcarrier wave frr to the antenna 45, and demodulator 41 for processing the received signal from the antenna 45.

The distance of communication between terminals A and B can be increased by the provision of one or more relay stations therebetween. Relay station C illustrates this arrangement but it is to be pointed out that a plurality of stations such as C can be used instead of a single relay station. The relay station C includes an antenna 50 to receive signals transmitted by antenna 33 and to transmit signals to the antenna 33. The relay station C also includes a second antenna 51 for communicating with the antenna of terminal station B. Signals received by the antenna from terminal A are applied through filter 52 to demodulator 53 wherein the subcarrier wave fro is derived from the carrier wave F1 and applied to the oscillator 54. The oscillator 54 is modulated by the subcarrier waves to produce a new carrier wave F3 which is radiated by the antenna 51.

Similarly, signals received by the antenna 51 from station B are applied through filter 55 to demodulator 56 in which the subcarrier wave in is derived from the carrier wave F2 and applied to the oscillator 57 for modulating the same so that a modulated carrier wave F4 is radiated by the antenna 50. The carrier waves, F1 and F2, received by the antennas 50 and 51 and the waves, F: and F4, transmitted thereby are preferably all at dif- 4 ferent frequencies although some duplication of frequencies may be possible. In the relay station C the oscillators 54 and 57 provide local oscillators for the demodulators 56 and 53 respectively, as in the terminal stations described above. Connection is made to the demodulators 56 and 53 through attenuators 58 and 59 which reduce the level of the oscillations. It is apparent from the above description that continuous simultaneous transmissions may be made from station A to station B, and from station B to station A, with the relay station C relaying the signals in both directions at the same time.

In Fig. 4 a modified relay station is illustrated including terminal stations A and B and relay station C1. The terminal stations A and B are identical with the terminal stations A and B of Fig. 3. The relay station C1, however, has a somewhat different system. Considering now the relay of signals from station A to station B through relay station C1, signals received by antenna 50 are passed through filter 60 to demodulator 61. The subcarrier wave fro is derived from the carrier wave F1 in the demodulator 61 and applied through filter 62 to heterodyne converter 63 which changes the frequency of the subcarrier wave to provide a new subcarrier wave in of a difierent frequency. The output of the converter 63 is applied to filter 64 which allows the new subcarrier wave frz to pass but blocks the original subcarrier wave fro. The new subcarrier wave is then applied to the oscillator 65 and modulates the carrier wave Fa produced thereby. The modulated carrier wave produced by the oscillator 65 is applied to the antenna 51 and radiated thereby. The output of the oscillator 65 is also applied through attenuator 66 to the demodulator 61 and thereby provides the local oscillations for demodulating the received wave as in the modifications described above. It will be apparent that when a single oscillator is used in a relay station as the transmitter and for providing local oscillations in the receiver, the transmitted and received wave must be modulated by different subcarrier waves. The filter 62 which passes the subcarrier wave fro but rejects the wave frz, and filter 64 which passes the subcarrier wave fra but rejects fro prevent regenerative action beca6u6se of energy which is passed through the attenuator The system for relaying signals transmitted from station B to station A is identical to that described above with signals transmitted by antenna 45 being picked up by antenna 51 and applied through filter 67 to the demodulator 68. The subcarrier wave in is derived from the carrier wave F2 by the demodulator 68 and passed through filter 69 to heterodyne converter 70 wherein it is converted to a subcarrier wave f1; of difierent frequency. The new subcarrier wave fr: is applied through filter 71 to the oscillator 72 which produces a modulated carrier wave F4 which is radiated by the antenna 50. A portion of the output of the oscillator 72 is applied through attenuator 73 to the demodulator 68 providing local oscillations for demodulating the received wave.

In Fig. 5 there is illustrated still a further radio relay system with the terminal stations A and B being generally identical to the terminal stations of Figs. 3 and 4, but with the relay station Cz being of different construction than the relay stations C of Fig. 3 and C1 of Fig. 4. The principal characteristic of the relay station C2 is that a single oscillator is provided for transmitting in both directions and for providing local oscllations for the receiving systems which receive from both directions. Referring to the drawings the oscillator 32 of the terminal station A provides a carrier wave at a frequency F1 modulated by a subcarrier wave fro. This carrier wave is transmitted by antenna 33 and picked up by antenna 50 at the relay station C2. The received wave is applied through filter 75 to demodulator 76 wherein the subcarrier wave fro is derived from the carrier wave F1. The subcarrier wave fro is passed through a filter 77 and is converted to a subcarrier wave in at a different frequency in the heterodyne converter 78. The output of the converter 77 is passed through filter 79 which passes the new subcarrier wave frr but rejects the subcarrier wave fro. The subcarrier wave In is then applied to the 0s. crllator 80 which operates at a frequency F2. The output of the oscillator 80 is applied to the antenna 51 from whrch rt rs transmitted to the next relay or terminal statron. Local oscillations are applied to the demodulator 76 from the oscillator 80 through attenuator 81. As

the oscillator is modulated by subcarrier frequencies the demodulator 76 will provide these subcarriers as well as the subcarrier fin but all of the subcarriers will be eliminated by the filter 77 except the subcarrier fro. The filters 77 and 79 must not have overlapping frequency ranges or singing might take place at a frequency which would pass through both.

The relay station C2 operates in a similar manner to relay signals received by antenna 51 and transmitted by antenna 50. The oscillator 42 at terminal B provides a carrier wave F3 which is modulated by a subcarrier wave I12. This carrier wave is radiated at antenna 45 and received by antenna 51. The received wave is passed through filter 82 to demodulator 83 which derives he subcarrier wave f12 therefrom. The subcarrier wa fur is passed by filter 84 and converted to a subcarrier wave h; of a different frequency in heterodyne converter 85. The newly derived subcarrier wave in is selected in the filter 86 and applied to the oscillator 80 for modulating the same. The oscillator 80 therefor produces a carrier wave F2 modulated by subcarrier wave I13 which 1s applied to the antenna 50 and received by antenna 33 at the station A. Local oscillations are applied to demodulator 83 from oscillator 80 through attenuator 87.

It will be apparent from the foregoing that the oscillator 80 is modulated by both subcarrier waves fii and fig so that the carrier wave F2 transmitted in both directions by antennas 50 and 51 is modulated by the subcarrier waves which are to be transmitted in the two directions. However, this causes no difficulty as the receivers at the terminals A and B are selective only to the subcarrier wave which are to be received and the undesired subcarrier wave will be rejected. That is, the subcarrier receiver 36 at terminal A will receive only the subcarrier wave in and the subcarrier receiver 41 at terminal B will receive only the subcarrier wave fir. In this system, the frequency of the carrier waves F1 and F3 must necessarily differ from the frequency of the carrier wave F2 by the intermediate frequencies of the demodulators 76 and 83. It is desirable for the purposes of standardization to have the intermediate frequencies of these demodulators the same although this is not necessary. The waves F1 and F3 may be at different frequencies if one of the waves (F1) is equal to the frequency of the wave F2 plus the intermediate frequency, and the other (F3) is equal to the frequency of wave F2 minus the intermediate frequency. The same carrier wave Fz may be transmitted in both directions by the antennas 50 and 51 without causing dilficulty.

The system of Fig. 5 may be simplified somewhat by properly selecting the subcarrier frequencies fro and 152 which are received at the relay station C2. By using adjacent frequencies for these subcarriers, the filters 77 and 84 may be replaced by a single filter which will pass both subcarrier frequencies. A single converter may then be used in place of converters 78 and 85 and a single filter in place of filters 79 and 86. A single att8e7nuator may also be used to replace attenuators 81 and Although the systems of Figs. 3, 4 and 5 have been described as transmitting a single subcarrier wave, it is obvious that a plurality of subcarrier waves can be simultaneously transmitted by these systems to provide'a larger number of communication channels. In the systems of Figs. 4 and 5 a plurality of subcarrier waves can be converted in the same converters and passed through the same filters. In such arrangements the frequencies of the subcarrier waves must be selected so that subcarriers on adjacent frequencies are transmitted together and can be passed by the same filters. As previously stated, the filters before and after the converters must not both pass any one frequency or regeneration around the loop circuit may take place.

It is seen from the above that simultaneous two way microwave communication systems have been provided in which a single oscillator provides the waves to be transmitted and local oscillations for demodulat ing the received wave. This is made possible by providing transmission and reception simultaneously at different frequencies and by the use of subcarrier waves of diflFerent frequencies or the transmitted and received waves so that the received waves can be selected. This arrangement simplifies radio communication systems operating in the microwave ranges and particularly simplifies relay systems as the large number of oscillators normally required is reduced by half. Since oscillators suitable for operation in the very high frequency and microwave ranges are very expensive and delicate, this is a very worthwhile saving.

Although certain embodiments of the invention which are illustrative thereof have been described, it is obvious that various changes and modifications can be made therein without departing from the intended scope of the invention as defined in the appended claims.

I claim:

1. A radio relay communication system for providing simultaneous two way communication on frequencies in the microwave frequency range including in combination, a pair of terminal stations and at least one intermediate relay station, said relay station including a pair of antenna means each of which is positioned to communicate with one of said other stations, a relay circuit interconnecting said antennas for providing transfer of signal in both directions therebetween, said relay circuit including transmitting means connected to both antenna means for providing a modulated carrier wave for transmission thereby, said transmitting means including means for producing subcarrier waves and a single oscillator for providing a carrier wave modulated by said subcarrier waves, a pair of receiving means individually connected to said antenna means for receiving carrier waves modulated by subcarrier waves of different frequencies than said transmitted subcarrier waves, and means for applying said modulated wave produced by said oscillator to both of said receiving means for converting the carrier waves received thereby to waves of lower frequencies.

2. A radio relay communication system for providing simultaneous two way communication on frequencies in the microwave frequency range including in combination, a pair of terminal stations and at least one intermediate relay station, said relay station including a pair of antenna means each of which is positioned to communicate with one of said other stations, a relay circuit interconnecting said antennas for providing transfer of signal in both directions therebetween, said relay circutit including transmitting means connected to both antenna means for providing a modulated carrier wave for transmission thereby, said transmitting means including means for producing subcarrier waves and a single oscillator for providing a carrier wave modulated by said subcarrier waves, a pair of receiving means individually connected to said antenna means for receiving carrier waves modulated by subcarrier waves of different frequencies from said transmitted subcarrier waves, means for applying said modulated wave produced by said oscillator to both of said receiving means for converting the carrier waves received thereby to waves of lower frequencies, said receiving means deriving said received subcarrier waves from said received carrier waves, means for converting said received subcarrier waves to waves of different frequencies, and means for applying said subcarrier waves so produced to said oscillator for modulating the wave produced thereby.

3. In a radio relay communication system for providing simultaneous two way communication on frequencies in the microwave frequency range and which includes a pair of terminal stations and at least one intermediate relay station, a radio relay station including in combination, a pair of antenna means each of which is positioned to communicate with another station, a relay circuit interconnecting said antenna means for providing transfer of signals in both directions therebetween, said relay circuit including transmitting means connected to both antenna means for providing a modulated carrier wave for transmission thereby, said transmitting means including single oscillator means for providing a carrier wave modulated by at least one signal wave, a pair of f receiving means individually connected to said antenna means for receiving carrier waves of frequencies different from that of the transmitted carrier wave, which 1 received carrier waves are modulated by signal waves of frequencies different from the frequencies of the signal waves modulating said transmitted carrier wave, and means for applying said modulated carrier wave produced by said oscillator means to said pair of receiving means for converting the carrier waves received thereby to waves of lower frequencies.

(References on following page) References Cited in the file of this patent UNITED STATES PATENTS 8 Trevor Feb. 13,

Beverage Nov. 8, Baum Feb. 7, 

